1. Field of the Invention
This invention relates to a noise-squelcing circuit for use with a signal receiving system that includes a phase-locked loop for demodulation of incoming modulated carrier signals. More particularly, the invention is directed to a noise-squelching circuit for an amplitude modulation (AM) receiver with a phase-locked loop to provide a demodulation carrier, the loop also provides an output signal from a low pass filter to control a muting circuit and allow demodulated signals to pass through the muting circuit only when the signal from the filter indicates a phase-locked condition with respect to an incoming signal.
2. The Prior Art
AM receivers have been known heretofore in which a phase-locked loop is used as a demodulator. In such receivers a voltage controlled oscillator included in the phase-locked loop produces a reference carrier signal that is locked to the frequency of an incoming modulated carrier signal. The reference carrier signal is applied to a synchronous detector with the modulated carrier signal to effect demodulation of the incoming signal. It has also been known to provide a muting circuit in such receivers to mute the noise that would otherwise pass through the demodulator as the receiver was being tuned from one station to another. Such muting circuits have been provided at the demodulated output signal side of the demodulator and have been controlled by a signal obtained by rectifying the intermediate frequency (IF) signal in the receiver. However, such circuits cannot positively discriminate between a noise signal in the IF amplifier and an information signal. Furthermore, it is not easy to adjust the muting level of the muting circuit due to the fact that the level adjustment cannot be done smoothly.
Accordingly, it is one object of the present invention to provide a noise-squelching circuit that includes a phase-locked loop in the demodulation section of a receiver to provide an identifiable signal to control a muting circuit.
Another object of the invention is to provide an AM receiver with a demodulator that includes a phase-locked loop that produces a reference carrier signal for synchronous detection of an incoming signal and further includes a low pass filter that produces an identifiable output signal for use as a muting signal when the loop is in phase-locked condition with respect to an incoming carrier signal.
Further objects, features, and advantages of the present invention will be apparent from the following description taken in connection with the accompanying drawing.